Resistors are commonly used in electronic circuits to inhibit the flow of electric current. Frequently, thin film resistors are combined with other semiconductor structures to make extremely compact, yet complex, circuitry. For example, thin film resistors may be used as part of an individual device, e.g., a resistor in a power transistor, or may be used in connection with a multiple number of other semiconductor devices to form complex integrated circuits and/or hybrids. For example, such thin film resistors may be used as current limiting and load resistors in amplifiers, as a part of resistor arrays, etc.
Thin film resistors generally consist of a thin film of resistive material deposited, such as by sputter deposition, on a layer of insulative material of a substrate. End contacts are formed relative to the resistive material. The end contacts or interconnections are then connected to other circuit components in a conventional manner.
Thin film resistors are generally characterized in terms of their sheet resistance and their temperature dependence. Sheet resistance is expressed in resistance per unit area (e.g., ohms per square (.OMEGA./sq)) and is equal to the bulk resistivity divided by the film thickness. Sheet resistance is a material property and is not dependent on the topology of a particular resistor. The resistance of a specific resistor is obtained by multiplying the sheet resistance by the ratio of the resistor length to width.
The temperature dependence of thin film resistors is described in terms of the temperature coefficient of resistance (TCR) which reflects the slope of the resistivity versus temperature curve. In other words, the TCR reflects the fractional change in resistance per unit change in temperature. Generally, it is expressed in parts per million change per degree centigrade (ppm/.degree. C.). The TCR may be positive or negative. Conventional film resistor materials typically have TCRs in the range of a few hundred to a few thousand ppm/.degree. C., positive or negative.
There are various criteria by which the quality of thin film resistors are evaluated. For example, it is generally desirable that a thin film resistor have a minimum thickness. When a thin film resistor is too thin, it may be unable to handle relatively large current densities during operation. Further, it is typically desirable that a thin film resistor have uniform thickness and properties to ensure consistency and stability. Also, it is generally desirable that thin film resistors have a target or intended sheet resistance. Yet further, it is normally desirable that thin film resistors have a very low TCR, or at least a TCR that is suitably matched to a particular application. For example, it may be desirable to have a TCR that is either positive or negative, or have a TCR that is zero. For example, a thin film resistor with a zero TCR does not vary in resistance as the temperature changes.
Various resistive materials have been used to form thin film resistors. For example, chromium diboride, silicon chromide, nickel chromium, etc. have been used to fabricate thin film resistors. However, although such resistive materials may be used to fabricate thin film resistors having relatively low sheet resistances, e.g., less than about 1.5 k.OMEGA./sq, these materials and others have not generally been suitable for attaining sheet resistances that are relatively high, e.g., 1.5 k.OMEGA./sq or higher. Particularly, such resistive materials have not been suitable for forming relatively high sheet resistance values due to the inherent properties of the resistive materials. For example, to attain relatively high sheet resistance using such resistive materials, the thickness of the thin film resistor formed is undesirably thin, e.g., less than 100 .ANG.. Not only are such film resistors undesirably thin and unable to handle relatively high current densities in operation, as previously described, but it is difficult to reproducibly form such undesirably thin film resistors. Further, uniformity of such thin resistors is problematic.
Relatively high sheet resistance characteristics are particularly important in various product areas. For example, electronic devices, which operate at relatively low power, generally require the use of thin film resistors with high sheet resistance. In particular, implantable medical devices generally operate at such lower power requirements and require high sheet resistance thin film resistors. Further, high sheet resistance also provides for a beneficial reduction in the size of a resistor.
Further, commonly used resistive materials do not provide for thin film resistors that have a TCR that is suitably controllable. For example, if a zero TCR is even achievable, it is generally zero TCR over only a very small temperature range. Further, for example, thin film resistors formed of such conventional resistive materials generally have a TCR that is not temperature independent. A temperature independent TCR is one in which the resistivity is a linear function of temperature over the temperature range of interest, e.g., -55.degree. C. to about +125.degree. C. For example, a parabolic variation of resistivity with temperature does not have a temperature independent TCR.
As such, thin film resistors having high sheet resistance are desired. In addition, a thin film resistor having a controllable or adjustable TCR, e.g., a zero TCR or a TCR that is temperature independent, are important for various applications, e.g., implantable medical device operation.
Table 1 below lists U.S. patents relating to thin film resistor fabrication techniques:
TABLE 1 ______________________________________ U.S. Pat. No. Inventor(s) Issue Date ______________________________________ 4,510,178 Paulson, et al. 9 April 1985 5,468,672 Rosvold 21 November 1995 ______________________________________
Further, other thin film resistors are described in the book entitled Thin Film Technology by Robert W. Berry (1979).
All references listed in Table 1, and elsewhere herein, are incorporated by reference in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Embodiments, and claims set forth below, at least some of the devices and methods disclosed in the references of Table 1 and elsewhere herein may be modified advantageously by using the teachings of the present invention. However, the listing of any such references in Table 1, or elsewhere herein, is by no means an indication that such references are prior art to the present invention.